Boiler cuts out

Postby willemm » Thu Jun 04, 2009 10:55 pm

My boiler serves both CH and/or hot water via pump and motorised valve. It's now on hot water only via timer, but is cutting out after only 2 mins, and wont fire up again for another 10 mins, then on again for another 2 mins. One hour of timer provides 10 minutes of heated water. Pump is noisy, but this pipe run (water only) is fairly short and 22mm so airlock seems unlikely. At 10 years old could it be a blockage? Header tank (about 25 litres) contained a reddish/brown sediment which can only have come from expansion pipe from boiler. System did have Fernox put in when new but various plumbing jobs may have weakened it over time.
All references to sludge say it is black but what I took out (I emptied and cleaned the H&E tank) was 300 m/l of sediment.
Any suggestions as to possible cause and remedy (other than getting plumber in) very welcome.
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Simply Build It

Postby htg engineer » Sat Jun 06, 2009 6:50 pm

Could be an airlock, could be a blockage, or could be the pump.

As you say the pump is noisy, I would try venting the pump first, if there's no air or is still noisy after the air is released I would look at changing the pump. Sometimes the impeller comes off the motor, therefore the water will not be circulated and the boiler will reach temperature quickly - which is happening in this case.

htg engineer
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Postby willemm » Sat Jun 06, 2009 11:08 pm

Thanks for response htg.

A new pump was fitted by plumber last September when problem the same then and it keeps coming back; re-appeared in February, went away again (after visit by plumber) until recently. A new 3-port valve diverter with motorised unit was also replaced earlier. As CH is now off, the problem seems to lie between boiler and pump. I removed large screw from front of pump and little water was passing. System is only 10 years old, serviced every year, and Fernox was put in it by installer so shouldn't be scaled up enough to cause a blockage.
I find it hard to believe that the 22mm feed between boiler and pump is blocked, and have bled vent in loft with only water coming out.
The only suspect point for blockage is tee joint off boiler output (about 4 metres along) where it drops down into airing cupboard and pump. Tee continues on to H&E tank.
If it's a blockage, why would problem keep going away, then come back. unless air locks are forming somewhere ? Can't make sense of this at all, and not sure my plumber can either.
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Postby willemm » Sat Jun 06, 2009 11:19 pm

Thanks htg
Pump was replaced by plumber last September with same problem. I have checked it but very little water comes out. There is a loft vent which I bled, but no air came out.
The same problem re appeared in February. Plumber managed to deal with it, possibly an air lock then as CH was suffering. But CH is now off, and boiler only has to serve hot water which is a short 4 metre run across loft where a tee takes pipe down into airing cupboard and pump. Tee continues to H&E tank.
This short run is all 22mm pipework, so hard to believe that it can have either a blockage or an airlock (or both!).
Puzzle is, why does it go away, then come back. If a blockage it will stay until unblocked, so could it still be an airlock, if so how to get at it ?
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Joined: Thu Jun 04, 2009 10:36 pm

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